Optical scatterometry, reflectometry, or ellipsometry methods can be used to measure grating profile shapes and critical dimensions that are significantly smaller than the measurement instrument's optical wavelength(s) and imaging resolution. With this technique, a grating sample's reflectivity in one or more diffracted orders (typically the zero order) is measured at multiple wavelengths, incidence angles, and/or polarization states, and a theoretical electromagnetic scattering model is fit to the measured data to determine grating parameters such as profile shape and line width. For example, FIG. 1 schematically illustrates reflectometer-type measurement instrument that can be used for grating measurement. Illumination from a source 101 transmits through a beam splitter 102, and is focused by an objective lens 103 onto a small focus spot 104 on a grating sample 105. Reflected radiation is diverted by beam splitter 102 toward a radiation-sensing detector 106. Reflectivity data is acquired over a range of wavelengths and is computationally processed to determine grating parameters such as line width, thickness, etc. The illumination system in FIG. 1 has a limiting aperture 107 whose size and shape determines the size and shape of the diffraction-limited focus spot 104 on grating 105. Typically, the focus spot must be large enough to cover multiple grating lines to achieve good measurement sensitivity. But if the grating is close to adjoining structures a large spot size may result in degraded measurement performance due to proximity effects. This tradeoff is illustrated in FIGS. 2A and 2B. These figures show a plan view of grating 105 comprising grating lines 201a, 201b, etc. of limited length. (The grating lines may actually be much longer, but the useful area for measurement may be limited due to other structures overlying or underlying the grating.) The limiting aperture, illustrated as circle 107, controls the size and shape of the focus spot 104. A large aperture results in a small focus spot which does not cover sufficiently many grating lines to achieve good measurement sensitivity (FIG. 2A). A small aperture results in a large focus spot covering many grating lines (FIG. 2B), but in this case the focus spot extends outside of the measurable grating region 105, resulting in degraded measurement performance.